1. Field of the Invention
The present invention generally relates to semiconductor devices, and more particularly to a semiconductor device having a base member on which a semiconductor element is mounted.
Recently, it has been required to provide semiconductor devices having a package structure which enables high integration, high operation speed, high power and low cost. A package structure of BGA (Ball Grid Array) type has been proposed to meet the above requirement and applied to various electronic devices. The BGA type is one of very attractive package structures.
2. Description of the Related Art
FIG. 1 is a cross-sectional view of a conventional semiconductor device 11, which has a package structure of a plastic BGA (hereinafter simply referred to as a PBGA). The semiconductor device 11 has a printed circuit board 12 having an upper mounting surface 12a to which a single semiconductor element 13 such as a semiconductor chip is fixed by die bonding or the like. The printed circuit board 12 has a lower mounting surface 12b opposite to the upper mounting surface 12a. A plurality of solder bumps 14, which function as external connection terminals, are disposed on the lower mounting surface 12b.
Given electrode patterns (not shown) are provided, by printing, on the upper mounting surface 12a of the board 12 and inner layer portions of the board 12b. Wires 15 are provided between the given electrode patterns formed on the upper mounting surface 12a and the semiconductor element 13, and electrically connect these parts together.
A plurality of through holes 16 are formed in the printed circuit board 12, and function to extend the electrode patterns electrically connected to the semiconductor element 13 to the lower mounting surface 12b of the board 12 and connect these electrode patterns to the solder bumps 14 formed thereon.
A sealing resin 17 hermetically sealing the semiconductor element 13 is formed on the upper mounting surface 12a of the printed circuit board 12 by potting (may be formed by a transfer molding process). The sealing resin 17 is provided to protect the semiconductor element 13, which is embedded therein.
However, the semiconductor device 11 of the PBGA type as described above has the following disadvantages. The sealing resin 17 is likely to flake off from the printed circuit board 12 and thus the reliability of the semiconductor device 11 is low. This is because the printed circuit board 12 is used as a member for mounting the semiconductor element 13 and the sealing resin 17 is provided on the upper surface thereof by the potting or transfer molding process. There is a poor bondability between the printed circuit board 12 and the sealing resin 17. Further, the sealing resin 17 is fixed only to the upper mounting surface 12a of the board 12 by the potting or transfer molding process, so that the mechanical strength obtained at the interface is weak.
Furthermore, the printed circuit board 12 has a low thermal conductivity, and is not capable of efficiently radiating heat from the semiconductor element 13. In addition, the printed circuit board 12 has poor wetproof, and low reliability particularly with respect to thermal stress.
Moreover, the semiconductor device 11 of the PBGA type does not have any electrodes (leads) used to confirm whether the solder bumps 14 are electrically connected to the pattern of the printed circuit board 12 appropriately. Hence, it is impossible to perform a test (called a Field Test) directed to check the electrical connections of the solder bumps 14 to a mounting board after the semiconductor device 11 is mounted on the mounting board.
Similar problems will be encountered in another type of semiconductor devices.
FIGS. 2 and 3 are diagrams of a semiconductor device 110 of having a package structure of QFP (Quad Flat Package) type. The semiconductor device 110 is generally made up of a semiconductor element 111, leads 112, a stage 113 and a resin package 14. The semiconductor element 111 is mounted on the stage 113, and electrode pads 115 are provided on the upper surface of the semiconductor element 111.
The leads 112 include respective inner lead portions 112a and outer lead portions 112b. The inner lead portions 112a are connected to the electrode pads 115 by wires 116. The outer lead portions 112b are shaped into a gull wing suitable for surface mounting. The resin package 114 hermetically seals the semiconductor element 111 and protects the same. The resin package 114 can be formed by transfer molding or the like. The inner lead portions 12a are provided so as to be embedded in the resin package 114. The outer lead portions 112b outwardly extend from the sides of the resin package 114. The semiconductor element 111 has the QFP type package, and hence the outer lead portions 112b outwardly extend from the four sides of the resin package 114.
However, the semiconductor device 110 as described above has the following disadvantages. As the semiconductor element 111 has a higher integration density, the leads 112 become finer and the pitch between the adjacent leads 112 become narrower. As a result, the mechanical strength of the leads 112 is decreased, which allows easy deformation of the leads 112. Hence, it is necessary to perform, before shipping, a lead test which confirms if the leads 112 are suitably provided to the semiconductor element 111. The above test is troublesome. Further, if deformed leads are found, a repairing process must be done to reshape the deformed leads. This is also troublesome.
Further, there is a difficulty in down-sizing of the semiconductor device 110 because the leads 112 outwardly extend from the sides of the resin package 114. Furthermore, it is difficult to efficiently radiate heat generated in the semiconductor element 111 because it is hermetically sealed by the resin package 114 and resin generally used to form the package 114 has a poor thermal conductivity.